Negative masking is the suppression of nocturnal locomotor activity that occurs when a mammal is exposed to light. On the one hand, it has been considered to be either an inconvenience for circadian rhythm assessment insofar as it can obscure underlying events. On the other, masking has been studied for its own sake and is known to be mediated by the same classes of photoreceptors that mediate light-induced phase shifts. In fact, although light-induced masking has been considered to be fundamentally different from light-induced phase shifts, the only data that actually speak to the issue come from two suprachiasmatic nucleus (SCN) lesion studies. The more careful of these supports the view that the SCN is necessary for each response and, by extrapolation, is the origin of both. This view is also consistent with substantial data showing that light and locomotor activity can mutually inhibit each other's effect on circadian phase. This proposal uses mice to address two issues fundamental to the circadian visual system. Behavioral analysis will allow comparison of the effects of millisecond light stimuli (number, intensity, inter-flash interval)on masking and phase shifts. An argument by analogy will be made in support of the proposition that these two responses are similarly modulated by light and, therefore, likely to be controlled by the same input pathway and retinorecipient nucleus (SCN). Transgenic mice (melanopsin deficient (Opn4-/-) and rod/cone degenerate (rd-/rd-)) will be used to eliminate function the various photoreceptors and determine the extent to which masking and phase shifts are mediated by the identical photoreceptors. They will also determine whether ipRGCs are a necessary part of the photoreception pathway to the SCN that mediates both phase shifts and masking, even if such cells do not contain melanopsin photopigment. Concluding studies will use a chemical, and knife cut, lesion and tract tracing methods to explicitly determine whether or not the SCN mediates both phase shifts and masking, and whether the same brain regions or pathways are involved in the phenomenon of light-induced sleep. This is particularly important because there is, as yet, no anatomical or physiological evidence that effects of light on masking, phase shifts or light-induced sleep can be distinguished at the level of SCN cells. The results are expected to greatly improve our understanding of the anatomy, physiology and function of the photic input pathway mediating circadian rhythm phase shifts, masking and light- induced sleep. The use of very brief (millisecond) photic stimuli will provide a novel approach for studying processes regulating those three behaviors. PUBLIC HEALTH RELEVANCE Physiological and behavioral circadian rhythms are synchronized to light signals by a pathway involving several different photoreceptor types. This project utilizes novel light stimuli, new test procedures and mouse models to discern which photoreceptors, neural pathways and brain regions contribute to rhythm timing and to the ability of light to suppress locomotion and induce sleep. Studies are also designed to promote basic understanding of visual health and normal function. In order to develop methods of clinical care for persons suffering from such problems as delayed phase sleep syndrome, seasonal affective disorder, inappropriate timing of sleep or other bodily rhythms, it is necessary to understand how photic stimuli act upon, and regulate, the circadian rhythm system, and how it relates to sleep and arousal.